Primary biliary cirrhosis (PBC) is a chronic disease characterised by progressive inflammatory obliteration of the intrahepatic bile ducts. The disease is marked by an autoantibody response to mitochondria.sup.1-4, originally identified using immunofluorescence.sup.5. With the reactant use of immunoblotting, specific proteins have been recognized as targets of the anti-mitochondrial antibodies (AMA) of PBC.sup.2,6,7. In particular, serum antibodies to a 70 kilodalton (kd) protein have been found in greater than 95% of patients with PBC but not in patients with other autoimmune liver diseases,.sup.2,8 ; two other proteins of 45 and 39 kd are less frequently detected in PBC sera.sup.1,2,9. The identity of each of these autoantigens has been unknown, as is the relationship of these antigens to the pathogenesis of the disease. However, the 70 kd antigen is highly conserved in evolution, being present in mammals, yeast and bacteria.sup.10 and it is therefore believed to have an important structural or biological function.sup.2.